The present invention relates to a process for the preparation of polymers and to an apparatus for performing this process.
For the manufacturing of polymers batch and semi batch procedures are known and applied (see Ullmann's Encyclopedia of Industrial chemistry, 5.ed on CDrom). For both types of batch processes the thermal process safety is a challenge as after the start of the mostly exothermal reaction only minor control regarding the conversion and heat release is possible. This means that it is very difficult to maintain a stable product quality resulting in broad tolerances of the product specifications.
A further disadvantage is that certain parameters of such a batch process are only valid for a specific equipment (combination of reactor, stirrer), so that an increase of productivity by using a larger reactor or alternatively the usage of a different reactor is not easily possible.
It is known that an emulsion/suspension polymerization may be performed in a continuous mode, whereby for example in a static mixer an emulsion is formed, and the polymerization reactions during this process result in a suspension of the product in an aqueous or organic solvent. Such a setup is described for example in DE19816886C2 or EP1439196A1. In both patents it is described that a monomer solution and a radical initiator have to be two different feeds that are mixed by a micro mixer. A polymerization may make use of a radical initiator to start the radical chain reaction. The radical initiator is activated by a temperature increase either prior or after mixing with the reaction mixture. In DE19816886C2 and EP1439196A1 the radical initiator is heated before the mixing, such that radicals are formed. Therewith, the monomers undergo a reaction to form the polymer during and directly after micro mixing.
The temperature increase needed for the start of the radical chain reaction may be applied by heating the reaction vessel or by leading the reaction mixture (or a part of the reaction stream) through a heat exchanger device. Continuous flow heating using a microwave is a well known alternative since years. Commercially available microwave flow heaters are able to generate between 100 W to 30 KW and more microwave power to heat a flow of liquid up to 350° C. and more. The advantage of microwave heating especially for a continuous process is that a uniform and exact temperature profile may be generated along the path which is heated. Furthermore the heat comes “from inside out” meaning that no temperature gradient from the wall of tube to the flowing media is present which may lead to deposition of solids and over reaction products at the inner side of the tube (see Microwaves in Organic Synthesis, Andre Loupy, Wiley-VCH, 2006 ISBN-13 978-3527314522). Therefore a microwave is very suitable to heat reaction mixtures and especially to activate radical initiators for polymerization reactions as they remain unactivated prior to the heating and will be activated in a very homogeneous and very fast manner. This also improves the safety situation as it is easily to “switch-on” or “switch-off” the reaction by control of the microwave irradiation. The benefits of having microwave activation for radical initiated polymerization are for example described by Bogdal et al. (Adv. Polym. Sci. 2003, 163, 193-263).
A drawback of the processes as disclosed in DE199816886C2 or EP1439196A1 is that the reaction conditions and the design of the setup have to be chosen very properly to control the reaction, to avoid depositioning of product on the wall of the equipment and to control the properties of the product as the reaction is started in non-equilibrium state, i. e formation of emulsion and reaction occur at the same time.